Downhole tools for earth boring and for other purposes, including rotary drill bits, are commonly used in bore holes or wells in earth formations. One type of rotary drill bit is the roller cone bit (often referred to as a rock bit), which typically includes a plurality of conical cutting elements (often referred to as cones or cutters) secured to legs dependent from the bit body. For example, the bit body of a roller cone bit may have three depending legs each having a bearing pin (otherwise referred to as a journal pin). A rotatable cone may be mounted on each of the bearing pins. The bit body also may include a threaded upper end for connecting the drill bit to a drill string. During drilling, the rotation of the drill string and the contact of cutter elements with rock produce rotation of each cone about its associated bearing pin. The weight on the bit together with the rotation of the cones thereby causes the cutter elements to engage and disintegrate the rock.
The roller cone bit may have a sealed bearing system with grease lubrication to extend the bearing life. These bits operate in an extremely hostile environment due to high and uneven loads, elevated temperatures and pressures, and the presence of abrasive grit both in the hole cuttings and the drilling fluid. This is particularly true when drilling deep bore holes. In addition, some rock bits such as those used in geothermal exploration as well as in some hydrocarbon-bearing formations are subject to corrosive chemical environments in the form of, for example, carbon dioxide and hydrogen sulfide. When the seal is compromised, the bearing degrades rapidly due to loss of lubrication and can result in catastrophic bit failure. Another factor that can lead to early bearing failure is the inability of the bearings to withstand changes in the moment of forces directed against the roller cone. For example, the side forces (e.g., forces that may arise from eccentrically contacting one side of the bore hole) may tend to cause cone cocking or misalignment, thereby, producing high contact pressure, leading to high wear rate, and contributing to early bearing failure. The wear in the bearings will aggravate the cone misalignment and displacements and results in high seal leakage, which accelerates the degradation process. In addition, the bearing's load carrying capacity may limit both the load that can be applied to the bit as well as the angular velocity at which the bit can be rotated, thereby establishing constraints on achievable penetration rates and feasible cutter designs.
In downhole motors and submersible pumps, bearing wear is the source of significant problems. The wear is predominantly caused by third particle abrasion and erosion due to the abrasive grits present in the fluid flow.
In view of the foregoing, improved mechanical joints for downhole tools would be desirable.